Zucker rats are used as a model for human obesity, a major health problem which occurs as result of an imbalance of energy intake and expenditure. The effect of cholecystokinin (CCK) and the octapeptide (OP-CCK), a proposed satiety hormone, on feeding behavior was analyzed in Zucker obese and lean rats. Individual chambers were equipped with a bar pressed to obtain Noyes pellets and with a drinkometer to measure licks for water. Data are encoded on a magnetic tape by a Massey-Dickenson acquisition system; software has been developed for analyzing feeding and drinking behavior. In lean rats the size of the first meal following a 6 hour fast was decreased by 20 U (13%, P less than .05) and 40 U (34%, p less than .05) CCK/kg and by 40 U (22%, p less than .05) and 80 U (51%, p less than .05) OP-CCK/kg. Meal duration and first intermeal interval after 80 U OP-CCK/kg were decreased (51%, p less than .05). In obese rats 20 U CCK/kg, in contrast to lean rats, increased first meal size (6%, p less than .05) and duration (12%, p less than .05). In obese rats as in lean rats 40 U CCK and OP-CCK/kg decreased size of first meal (34%, p less than .05 and 35%, p less than .05, respectively) and 80 U/kg OP-CCK decreased first meal size (51%, p less than .05) and duration (50%, p less than .05). In the non-water-deprived rats drinking during the first meal was decreased with 40 U CCK (40%, p less than .05), but not affected by OP-CCK.